In general, a rod-like workpiece having a ball portion at one end, such as a ball joint, is manufactured by a cutting and grinding process. As shown in FIG. 12, a rod member 50 has a narrow diameter portion 51, a flange with a hexagonal cross-section 52, and a cylindrical tip portion 53. The rod member is held at the narrow diameter portion 51 by a chuck of a numerically controlled (NC) machine (not shown in the figure). Then, as shown in FIG. 13, a sloped surface 54 and an arched surface 55 are formed by a cutting and grinding process when a cutting point of a bite (not shown in the figure) radially approaches the cylindrical tip portion 53 near the flange 53 and the bite is moved axially as it changes its radial position with respect to the cylindrical tip portion.
By moving the bite radially and axially towards the tip of the cylindrical tip portion 53, a hemisphere 56 is formed at the tip of the cylindrical tip portion 53 by a cutting and grinding process as illustrated in FIG. 14. As a result, a ball joint 58 is formed at the end of the rod member. Alternatively, in this cutting and grinding process using an NC machine, the ball joint 58 can be formed by cutting and grinding the sloped surface 54 and the arched surface 55 after the hemisphere 56 is formed.
It is possible to form a ball portion 57 having desired shape and size by the cutting and grinding process for making the ball joint 58 described above. However, the hemisphere 56 and the arched surface 57 of the ball portion 57 need to be polished to increase surface roughness by a rolling round die having a varnished surface that has an arced cross-sectional shape and is coated with a super-hard alloy layer. Performing this process for each ball joint increases manufacturing time and cost. Moreover, a lot of scrapes are produced from the cutting and grinding process, and a complicated program is required for setting values for controlling the NC machine.
Therefore, the present invention is directed to address one or more of the above problems and to provide a rolling die that is capable of making a rod with a ball portion in a time and cost effective manner and with high precision and a method for making such a rod.